1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery for use in a high rate having superior battery characteristics under low temperature.
2. Description of the Related Art
A non-aqueous electrolyte secondary battery represented by a lithium ion secondary battery is a power source having a high energy density and a high voltage. In view of this, the non-aqueous electrolyte secondary battery is utilized as a main power source of a mobile device such as a mobile communication device or a portable electronic device. Also, in recent years, the non-aqueous electrolyte secondary battery has been used as an automobile power source or a power source for a DC-driven large-sized machine tool. In view of this, there is a demand for a compact and lightweight lithium ion secondary battery capable of fast charging and large current discharging.
On the other hand, the lithium ion secondary battery has to overcome the problems resulting from overcharging, external short-circuiting, or internal short-circuiting. In particular, lithium metal deposition may likely appear on a negative electrode surface at the time of fast charging or large current discharging. As a result, large exothermic reaction may occur in the lithium ion secondary battery during a nail penetration test, which is one of reliability tests, for instance. As a measure for the problem, a current control means using a voltage and/or a temperature is provided in an assembled battery. Also, a PTC thermistor and a current interruption mechanism are equipped in a unit cell battery.
There is a case, however, that removal of the PTC thermistor as a high resistor is required in order to secure a higher power from the lithium ion secondary battery. In such a case, the lithium ion secondary battery lacks one mechanism as a countermeasure against the lithium metal deposition. It is necessary to devise the material for internal parts of the battery and the battery layout to make up for the loss. From this point of view, the present inventors have considered use of a non-aqueous electrolyte solution that is less likely to cause lithium metal deposition on a negative electrode surface even in a high power condition.
Use of a non-aqueous electrolyte solution in a non-aqueous electrolyte secondary battery is proposed e.g. in Japanese Unexamined Patent Publications No. 7-14607 (D1) and No. 7-45304 (D2), although the non-aqueous electrolyte solutions disclosed in the publications are not used to suppress lithium metal deposition in a high power condition. D1 discloses improvement on cycle characteristic by using a non-aqueous electrolyte solution containing a mixed solvent of ethylmethyl carbonate (hereinafter, called as “EMC”) and dimethyl carbonate (hereinafter, called as “DMC”). D2 discloses improvement on discharge characteristic under a high temperature environment by using a non-aqueous electrolyte solution containing a mixed solvent of ethylene carbonate (hereinafter, called as “EC”), EMC, and DMC. Use of the non-aqueous electrolyte solutions containing the mixed solvents as disclosed in the publications could be proposed to suppress lithium metal deposition in a high power condition.
However, according to a detailed investigation by the inventors, use of the non-aqueous electrolyte solutions proposed in the publications failed to suppress lithium metal deposition even at room temperature if the secondary batteries carrying the non-aqueous electrolyte solutions are used in severe specifications (hereinafter, sometimes called as “high rate specifications”) such as a continuous discharge with a large current of 0.1 hour rate, or a constant power discharge with a high power of 100 W or more, or a fast charge with a large current of 0.5 hour rate, which is required to the battery for use in an automotive vehicle or a large-sized machine tool. Therefore, it is presumed that the tendency toward the lithium metal deposition will become pronounced in charging or discharging the battery at a low temperature of 0° C. or below, which would be a case when an automotive vehicle or a large-sized machine tool loaded with the battery is used in cold climates.
Specifically, the investigation by the inventors confirmed that neither the battery using the non-aqueous electrolyte solution, as disclosed in D1, satisfying the formula: 3/10≦(M+D)/T≦7/10 where T is the total volume of the non-aqueous solvents, M is the volume of EMC, and D is the volume of DMC, nor the battery using the non-aqueous electrolyte solution, as disclosed in D2, with volume ratios of EC, EMC, and DMC relative to the all non-aqueous solvents being respectively from 30 to 50% for EC, 10 to 50% for DMC, and 10 to 50% for EMC, has satisfactorily succeeded in solving the aforementioned problem under a large current discharge of 0.07 hour rate at room temperature. Further, the result leads to an estimation that the battery will cause the same or similar problem in use under the low temperature condition.